The term “noncoordinating anion” is now accepted terminology in the field of olefin and vinyl monomer polymerization to indicate an anion that stabilizes (stabilizing anion) a transition metal cation. This cation is generally accepted to be the active site in Ziegler-Natta catalysts. For example, the polymerization may be a coordination, insertion or carbocationic polymerization. See, e.g., EP 0 277 003, EP 0 277 004, U.S. Pat. Nos. 5,198,401 and 5,278,119, and Baird, Michael C., et al, J. Am. Chem. Soc. 1994, 116, 6435-6436, the disclosures of which are expressly incorporated herein by reference in their entireties for purposes of U.S. patent practice. NCAs are described to function as electronic stabilizing cocatalysts, or counterions, for essentially cationic metallocene complexes that are active for polymerization. The term “noncoordinating anion” as used herein applies both to truly noncoordinating anions and coordinating anions that are at most weakly coordinated to the cationic complex so as to be labile to replacement by olefinically or acetylenically unsaturated monomers at the insertion site.
In solid insulations, dielectric loss under alternating current (ac) field conditions commonly arises from relaxation processes associated with dipole orientation (polarization) and the movement of free charge carriers such as ions or electrons (conduction). The most commonly used parameter for expressing the dielectric loss of an insulator is the dissipation factor (tan δ). Dielectric tan δ, analagous to dynamic mechanical loss tan δ, is a measure of the ratio of the energy dissipated to the energy stored during a complete cycle of loading and unloading. However, in this case the load is an ac voltage rather than an oscillating mechanical strain. Ion conduction processes can lead to significant power losses in insulating materials if ionic charge carriers are present.
Electrical insulation applications are generally divided into low voltage insulation, which are those applications which generally involve less than 1,000 volts (1K volts), medium voltage insulation applications which generally range from 1,000 volts to 35,000 volts, and high voltage insulation applications, generally above 35,000 volts
Typical power cables such as those made for medium voltage applications include one or more conductors in a core that is generally surrounded by several layers that can include a first polymeric semi-conducting shield layer, a polymeric insulating layer and a second polymeric semi-conducting shield layer, a metallic tape and a polymeric jacket. A wide variety of polymeric materials have been utilized as electrical insulating and semi-conducting shield materials for power cable (e.g., building wire, electric motor wire, machinery power wire, underground power transmitting cable, and the like) and numerous other electrical applications.
In elastomers or elastomer-like polymers often used as one or more of the polymer members in electrical devices such as, e.g., power cables, ethylene/alpha-olefin/non-conjugated diene elastic polymer materials that have come into wide use usually include ethylene, an α-olefin such as, e.g., propylene, and a non-conjugated diene such as, e.g., 5-ethylidene-2-norbornene (ENB), 1,4-hexadiene, 1,6-octadiene, 5-methyl-1,4-hexadiene, 3,7-dimethyl-1,6-octadiene, and the like. Such polymers made with conventional Ziegler-Natta catalysts usually provide a good insulating property for power cables.
In comparison to conventional Ziegler-Natta catalysts (such as those based on titanium and vanadium transition metal compounds), metallocene-based catalysts offer a number of advantages that would make the latter appear to be even more suitable for the production of polymers and in particular, polyolefins to be used for electrical devices than the former. For example, due to the usually much higher activity of metallocene-based catalysts, the polyolefin made therewith can be expected to contain much less catalyst (metal) residue (“ash”) than a corresponding Ziegler-catalyzed polyolefin. This reduced ash content translates into a reduced number of metal ions in the final polymer that can contribute to electrical conductivity. Furthermore, in order to reduce the relatively high residual transition metal (e.g., Ti and/or V) content of Ziegler-catalyzed polymers, these polymers are often washed by slurrying them in a suitable aqueous liquid. To make this washing operation effective, slurrying aids such as calcium stearate are added to the aqueous liquid to keep agglomeration of polymer particles at a minimum. As a result thereof the washed polymer, while having a reduced transition metal content, contains ions derived from the slurrying aid (e.g., calcium ions) that adversely affect the insulating properties of the polymer.
Since metallocene-based catalysts overcome the above-discussed problems associated with the less active, conventional Ziegler-Natta catalysts, metallocene-catalyzed polyolefins would be expected to exhibit lower dielectric loss and, therefore, even better insulating property than polyolefins made by Ziegler catalysts. However, polyolefins made with catalysts derived from a metallocene catalyst precursor compound and a cocatalyst (activator) compound which includes a typically used NCA such as tetrakis(pentafluorophenyl)borate, B(C6F5)4−, show a higher dielectric loss than conventional Ziegler-Natta catalyzed polyolefins, making the former less suitable for electrical insulation purposes.
In the case of NCA-activated metallocene-catalyzed polyolefins ionic cocatalyst residues are known to be present in the polymers. These ions are believed to be major contributors to the high dielectric loss observed in these polymers. One way to avoid this problem in polymers to be used for electrical devices is to use metallocene cocatalysts that can easily be decomposed into uncharged species upon completion of the polymerization. A typical example of such a cocatalyst is an alumoxane such as methylalumoxane (MAO) which can easily be hydrolyzed to form methane and aluminum hydroxide (see, e.g., U.S. Pat. No. 5,246,783, incorporated herein by reference in its entirety for purposes of U.S. patent practice). However, while with an NCA such as B(C6F5)4− the molar ratio of metallocene transition metal (e.g., Zr) to NCA can be kept close to 1:1, a relatively large excess of alumoxane Al over the metallocene transition metal (e.g., 10:1 and higher) is needed to achieve high catalyst activity, thereby increasing the amount of catalyst residue (in particular, Al) in the resulting polymer. One of the reasons for the lower amount of NCA needed to activate the metallocene in comparison to an alumoxane is believed to be that the NCA is a stoicheometric activator (1 mole NCA:i mole metallocene). The stability of the NCA causes a problem after completion of the polymerization in that unlike an alumoxane, an NCA such as B(C6F5)4− is very hard to decompose, i.e., it cannot easily be converted into uncharged species that do not compromise the electrical insulation properties of the polymer containing them.
It is thought that the presence of stable anions, even in very low concentrations, cause dielectric loss due to their high mobility within the polymer interfaces. In view of the foregoing, there is a need for an NCA cocatalyst that, while showing a high stability, does not later cause any significant conductivity problems in the finished polymer. It would, thus be desirable to have available polyolefins made by an unsupported, highly active NCA containing catalyst system (such as a metallocene-based catalyst system) which show a dielectric loss that is low enough (i.e., have a sufficiently low tan delta) for making them suitable for use in electrical devices, e.g., for power cable applications.